1. Field of the Invention
The present invention relates to an electric power steering apparatus, and particularly relates to the electric power steering apparatus which is capable of being continuously operated even if a current detector for detecting a motor current breaks down.
2. Prior Art
Electric power steering apparatuses, that assist steering devices of automobiles and vehicles by means of torque of motors, apply driving forces of motors as assist forces to steering shafts or rack shafts by means of transmission mechanisms such as gears or a belt via reduction gears. Since such conventional electric power steering apparatuses generate assist torque (steering assist torque) accurately, motor current is controlled to be feed-backed. The feedback control is for adjusting a motor applying voltage so that a difference between a current control value and a motor current detected value becomes small or zero, and the motor applying voltage is adjusted generally by adjusting a duty ratio of PWM (pulse width modulation) control.
A general constitution of the electric power steering apparatus is explained with reference to FIG. 1. A shaft 102 of a steering handle 101 is joined to a tie rod 106 of a steering wheel via reduction gears 103, universal joints 104a and 104b and a pinion rack mechanism 105. The shaft 102 is provided with a torque sensor 110 that detects steering torque of the steering handle 101, and a motor 120 that assists a steering power of the steering handle 101 is connected to the shaft 102 via the reduction gears 103. An electric power is supplied from a battery 114 via an ignition key 111 and a power source relay 113 to a control unit 130 that controls the power steering apparatus. The control unit 130 calculates a current command value Iref of an assist command based on a steering torque T detected by the torque sensor 110 and a vehicle speed V detected by a vehicle speed sensor 112, and controls an electric current to be supplied to a motor 108 based on the calculated current command value Iref.
The control unit 130 is composed of mainly a CPU (including a micro processor unit or a micro controller unit), and a general function which is executed by a program in the CPU is shown in FIG. 2. For example, FIG. 2 does not show a current command value computing unit as hardware independent from a current command value computing section 204, but shows a current command value computing function to be executed in the CPU.
The function and the operation of the control unit 130 are explained. The steering torque T which is detected by the torque sensor 110 and is inputted and the vehicle speed V detected by the vehicle speed sensor 112 are inputted into the current command value computing section 204, so that the current command value Iref is calculated. On the other hand, a motor current I detected by a current detector 205 is feed-backed to a subtracting section 207, and a deviation “ΔI=Iref−I” from the current command value Iref is calculated. The deviation ΔI is inputted into a proportional-integral control section (PI control section) 208 as one example of a current controller, and a voltage command value Vref is calculated. A PWM control section 210 inputs the voltage command value Vref therein, and transmits a PWM signal determined based on the voltage command value Vref to an inverter circuit 211. The inverter circuit 211 supplies a motor current I to the motor 108 based on the PWM signal.
The above explains a basic control system of the feed-back control with respect to the motor of the electric power steering apparatus. In the feed-back control, it is an important requirement that the motor current can be correctly detected. As vehicles become larger, however, handle steering by a human power is more difficult, and thus even if the current detector breaks down, it is desired that the assist of the electric power steering apparatus is continued. In the case where the current detector breaks down, therefore, a control system that switches the control into a feed-forward control without using the motor current detected by the current detector so as to continue the assist of the handle steering is used as the control system that continues the assist as disclosed in Japanese Patent Application Laid-open No. 10-167086 A, Japanese Patent Application Laid-open No. 11-49002 A and Japanese Patent Application Laid-open No. 2002-87304 A.
The conventional control system for continuing the assist, however, relinquishes the feed-back control and switches the control into feed-forward control, but the feed-forward control has problems that the feeling of steering is inferior and an excess current which is not less than rated current might flow in the motor or the like in comparison with the feed-back control.